This invention relates to a power source circuit device for monitors and host computers and, more particularly, to an improved power source circuit device separately disposed in a monitor and an associated host computer from which the DC power source is directly supplied to the associated monitor so that frequently experienced interferences in the monitor are completely eliminated.
With the rapid development of computer science and information processing, monitors have become an increasingly demanded product the world over. Unlike television sets, with which a dynamic picture presented on the screen is usually used for recreational purposes, with the looker's eyes to be kept at a proper distance, computer monitors are used to view information presented on the screen and the user's eyes have to be kept at a close distance. Therefore, the information displayed on a monitor screen must be clear and stable; otherwise, it will pose a serious problem to the user's eyes. However, as conventional monitors are usually provided with a local power source circuit arrangement for being electrically connected to an external AC power source and obtaining a DC power therefrom, interference of a low-frequency magnetic field from a transformer disposed in the local power source circuit will affect the picture on the screen of the monitors, an example of this is the commonly encountered "water flowing ripple" interference, which is harmful to the user's eyes.
The above-mentioned monitors are shown in FIG. 1. Therein a known monitor MN1 is functionally connected to a host computer PC1 through a signal line SG1. The local power source circuit disposed in the monitor MN1 includes a step-down transformer T01 with its primary coil terminals respectively connected to the input terminals A1 and A2 which are adapted to be connected to an external AC power source, and with its secondary coil terminals separately coupled with a bridge rectifier D1 and a filtering capacitor C1 for providing required DC power supply to the monitor MN1. This known power source arrangement often suffers from low-frequency interference caused by both the magnetic field produced by the step-down transformer T01 and the synchroneous signals generated in perpendicular direction from the interface card within the monitor MN1. Consequently, "water flowing ripple" interference is present on the screen. Although certain measures have been taken to prevent this low-frequency interference problem, such as wrapping a silicon sheet around the transformer T01 for isolating the magnetic field thereof or changing the direction and location of the transformer T01, no significant improvement has been achieved by taking such steps.
Shown in FIG. 2 is another known monitor MN2 having its power source circuit arrangement disposed therein and electrically connected to a host computer PC2 through signal line SG2. As can be seen in FIG. 2, the external AC power source is input into a bridge rectifier D2 through input terminals B1 and B2, and rectified power is supplied to the transformer T02 and the base of transistor Q1 from which a high-frequency switching power supply is rectified and filtered before being fed into the monitor MN2. Although this high-frequency switching power supply has solved the low-frequency interference problem suffered by the prior art shown in FIG. 1, the harmonic waves generated along with the high frequency switching power supply of the power source circuit can hardly be eliminated therefrom. As a result, other kinds of interference, such as "snowing-like" high-frequency interference, often appear on the monitor screen. Of course, this kind of interference can be removed by improving the hardware design of the monitor; however, the manufacturing costs are increased by taking such steps.